Prefer "mov reg, wzr" over "mov reg, #0" #64740
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Tagging subscribers to this area: @JulieLeeMSFT Issue Detailsit's slightly more efficient. int Test() => 0;; Assembly listing for method Runtime:Test():int:this
- mov w0, #0
+ mov w0, wzr
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@dotnet/jit-contrib simple change, no size diffs, quite a few text diffs |
| GetEmitter()->emitIns_R_I(INS_mov, size, reg, 0 ARM_ARG(flags)); | ||
| #elif defined(TARGET_ARM64) | ||
| GetEmitter()->emitIns_Mov(INS_mov, size, reg, REG_ZR, /* canSkip */ true); |
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This seems like a more general optimization (there are many Arm64 instructions that support ZR or Immediate). Is there something we can/should be doing in emit instead to automatically recognize I == 0 and convert it to REG_ZR to ensure that this optimization always lights up and that devs don't need to remember to specially handle it in every scenario?
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@tannergooding I think we already do it (e.g. #52269), just not sure how this one was skipped.
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I'll take a look at r2r dump corelib to find #0
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That's just handling containment in lowering which isn't quite the same as something like this (which is the result of we need zero and can't contain)
I was referring to doing basically a peephole in emit that says "you're passing #0 but this instruction also supports REG_ZR, so fix it up there.
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By that PR I meant that we already do it since a simple change that just enabled containing resulted in ZR everywhere and huge diffs
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I've just checked all functions in R2R'd corelib - we only use #0 for cmp and movn now.
There are some cases where cmp reg, #0 is followed by a branch which most likely can be folded to "branch if zero" instructions but it's not related
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movn reg, #0 is interesting though
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it's slightly more efficient.